MongoDB - Update an object in nested Array

{
  "_id": "xPBc4By8FemDwTPqH",
  "u": {
    "_id": "6PoZawHZcQz4Gwzcv",
    "username": "michael"
  },
  "friends": [
    {
      "u": {
        "_id": "eGqDjAjjtYADbuSnn",
        "username": "michael",
        "name": "michael"
      }
    },
    {
      "u": {
        "_id": "k4gKCGwYryXDMMHvs",
        "username": "joyce",
        "name": "joyce"
      }
    }
  ]
}

I want to update the name of "friends.u.username": "michael" 's name is "hello", how I need to do it.

Apply the $set operator together with the $ positional operator in your update to change the name field.

The $ positional operator will identify the correct element in the array to update without explicitly specifying the position of the element in the array, thus your final update statement should look like:

db.collection.update(
    { "friends.u.username": "michael" }, 
    { "$set": { "friends.$.u.name": "hello" } }
)

You can use $set operator.

> db.test.update({"friends.u._id":"eGqDjAjjtYADbuSnn"},{$set:{"friends.$.u.name":"hello"}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Below should work fine as its tested

First check the current value of the array.

db.test.findOne({"friends.u.id" : "eGqDjAjjtYADbuSnn"},{"friends.u.name":1})

Now fire the update command

db.test.update({"friends.u.id" : "eGqDjAjjtYADbuSnn"},{$set:{"friends.$.u.name":"hello"}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })

Now check the results again to validate the update values

db.test.findOne({"friends.u.id" : "eGqDjAjjtYADbuSnn"},{"friends.u.name":1})

Hope this helps.

If you are using python, I created this function to generate the new update:

def generate_set_to_update_document(field: str, changes: dict) -> dict:
    new_set = {}
    for change in changes.keys():
        new_set[f"{field}.$.{change}"] = changes[change]
    return new_set